Traditionally, the natural gas industry has relied on gas chromatography to analyze the energy content or heating value of natural gas. The cost of installing, operating, and maintaining gas chromatographs is high.
As an alternative to gas chromatography, the following patents describe a new method for determining natural gas properties, such as heating value, molecular weight, and density: U.S. Pat. No. 6,604,051, entitled “System and Method to Determine Thermophysical Properties of a Multi-Component Gas”, U.S. Pat. No. 6,704,660, entitled “System and Method to Determine Thermophysical Properties of a Multi-Component Gas at Arbitrary Temperature and Pressure”, U.S. Pat. No. 6,850,847, entitled “Device for Determining Thermophysical Properties of a Multi-Component Gas at Arbitrary Temperature and Pressure, and U.S. patent application Ser. No. 10/770,768, entitled “Inferential Determination of Various Properties of Multi-Component Gases”. The method is based on the idea that natural gas properties can be inferentially determined from model equations having coefficients derived from a set of reference gases. More specifically, relatively simple measurements of the gas temperature, pressure, and sound speed, as well as the diluent concentrations for nitrogen and carbon dioxide are all that are required as inputs for the inferential algorithm.
This new technology provides substantial cost savings, with no appreciable loss in measurement precision, when used in place of conventional gas chromatographs. Also, because of its expected cost effectiveness, this technology can be deployed at many field measurement sites where gas chromatography is not economically viable. Although its primary application is expected to be for natural gas pipeline measurements, the inferential algorithm is useful for any gas mixture containing carbon dioxide and/or nitrogen diluents.
As stated above, one of the inputs for the inferential algorithm is the nitrogen concentration of the gas mixture. No simple, accurate and inexpensive method for determining nitrogen concentration in natural gas currently exists. However, instead of attempting to directly measure nitrogen, alternative methods have been developed for inferential measurement. For example, U.S. Pat. No. 6,804,610, entitled “Indirect Measurement of Nitrogen in a Multi-Component Gas by Measuring the Speed of Sound at Two States of the Gas”, describes a method of inferentially determining nitrogen concentration in a natural gas mixture by using the above-described inferential algorithm, applied at two different thermodynamic states. The technique is referred to herein as the “two-state” inferential gas analysis algorithm.